[OpenCL] Add builtin header TableGen emitter

Add an emitter to produce something similar to opencl-c.h from the
OpenCL builtin descriptions in OpenCLBuiltins.td

This only adds the emitter, without any direct use of it.  This allows
opencl-c.h additions to be generated from the builtin descriptions by
manually invoking `clang-tblgen -gen-clang-opencl-builtin-header`.

Differential Revision: https://reviews.llvm.org/D104040